Motion Blur at different frames
From what I have observed, when motion blur is enabled and Cycles is the renderer, Blender steps back a frame to compute the motion difference between the frame being rendered and the frame that came before - you can see this happening in the info line at the top of the UV/Image Editor when rendering is underway - so long as you leave all settings unchanged Blender should deliver consistent motion blur over whatever frame range you set - alternatively you can add a one-frame "head" and "tail" to your frame range to make sure you definitely get all frames properly rendered across the breaks in your sequence